We investigated the actions of the endogenous opioid tetra-peptide endomorphin 1, a selective mu-opioid receptor agonist, on oxytocin and vasopressin cell activity in vivo and in vitro. The activity of antidromically-identified supraoptic nucleus cells were recorded from urethane-anaesthetized female rats. The firing rates of both oxytocin and vasopressin cells were reduced by intracerebroventricular endomorphin 1 (5 - 100 pmol); this inhibition was prevented by intravenous naloxone (5 mg kg(-1)). A second group of rats was infused intracerebroventricularly with endomorphin 1 (27 pmol min(-1)) over 5 days. The firing rates of oxytocin and vasopressin cells in endomorphin 1 pre-treated rats were similar to those of endomorphin 1 naïve rats, indicating tolerance to the inhibitory effects of endomorphin 1. Intravenous naloxone induced similar modest and transient increases in the firing rate of oxytocin cells in endomorphin 1 pre-treated rats and endomorphin 1 naïve rats, indicating that endomorphin 1, unlike the mu-opioid alkaloid agonist, morphine, does not induce mu-opioid dependence in these cells. In vitro, whole-cell current clamp recordings were made from supraoptic nucleus cells in superfused coronal hypothalamic slices from young female rats. Endomorphin 1 (100 nM) inhibited the firing rate of oxytocin cells but had no significant effect on vasopressin cells at up to 10 microM. Inhibition of oxytocin cells was reversed by naloxone, and remained when synaptic transmission was blocked by superfusion with low Ca(2+)/Co(2+)-containing medium. Thus, endomorphin 1 directly inhibits oxytocin cells but inhibits vasopressin cells by indirect actions. Chronic endomorphin 1 administration induces mu-opioid tolerance in oxytocin and vasopressin cells but not mu-opioid dependence in oxytocin cells.